In the conventional diode-pumped solid-state laser, the profile of the exciting source (a high-power laser) is a strip of divergence (1 .mu.m in width and 200.about.500 .mu.m in length). Therefore, imaging elements such as coupling lens sets must be disposed between the pumping diode and the laser crystal to make the profile more symmetrical and direct the emanating beam to focus on the laser crystal. However, there are two alignment operations required for using coupling lens to emanate the beam from the pumping diode into the laser crystal. First of all, the imaging elements must be aligned corresponding to the output facet of the pumping diode. Second, the laser crystal must be properly aligned with diode-lens combination. These two operations need an extra adjusting motion along three orthogonal axes for the diode-pumped solid-state laser with a good output power. However, the alignment operation is relatively complicated and expensive.
In general, a diode-pumped solid-state laser includes a flat coupling mirror and a concave coupling mirror, both of which are disposed at two ends of the laser crystal respectively. It needs to precisely aim the focal point of the concave mirror and then adjust the slant angles of these two coupling mirrors. Therefore, an easy laser system is quite in demand and necessary in the practical applications.
A butt-coupled diode pumped laser has been disclosed in U.S. Pat. No. 4,848,815 to eliminate the complicated alignment operation in the pumped-diode solid-state laser as described above. The beam emanating from the pumping diode has a divergent light surface of strip shape. Consequencely, when the laser crystal is excited by the pumping beam, a high-order transverse mode of oscillation is generated and a single transverse laser with lower output power is obtained. Furthermore, the pumping diode is butt-coupled to the laser gain medium so that the heat emission from the pumping diode will decrease the performance and efficiency of the laser.
In order to obtain a single longitudinal-mode laser output, a microchip served as a cavity has been disclosed by Zayhowski in Opt. Lett. Vol. 15, No. 8, pp. 431-433, 1990. However, when the laser crystal is end pumped, a thermal lens effect will be generated because the length of the cavity is so short (about 1 mm) that the mode-to-pump size ratio is less than 0.6 resulting in high-order transverse mode of oscillation. In addition, other devices such as frequency-doubling crystals and Q-switches can not be disposed in the cavity so that its application is restricted.
It is obvious that the above-described diode-pumping solid-state lasers have some drawbacks as follows:
(1) It is relatively complicated to set up the laser system; PA1 (2) The performance and efficiency of the butt-coupled diode-pumped solid-state laser will be decreased by the heat emission from the pumping diode; PA1 (3) The cavity with a short length for generating a single longitudinal-mode laser will be disturbed by the thermal lens effect.